I did a little hunting and found this site:
http://www.kluug.net/delphitwain.php

And there is this comment:
TDelphiTwain 1.4 is not a TComponent descendand any more – you can’t install it into the Delphi pallete and use it as a non-visual component any more. You have to use it from code only and free it by yourself (see the DEMO).